The present invention relates to the field of automatic transmissions; more particularly, relates to the field of automatic transmissions with torque converters which are equipped with lock up clutches; and, yet more particularly, relates to the field of hydraulic fluid pressure control systems for such lock up clutches.
Various automatic transmissions for automotive vehicles are known in various forms. Such an automatic transmission conventionally includes a gear transmission mechanism which provides a plurality of speed stages and which is set to its various speed stages by selective supply of actuating hydraulic fluid pressures to various hydraulic fluid pressure activated friction engaging mechanisms comprised with the gear transmission mechanism such as multi plate clutches and multi plate brakes, said selective supply of actuating hydraulic fluid pressures being provided from a hydraulic fluid pressure control system, of which many forms are well known. Such a hydraulic fluid pressure control system typically requires a supply of line hydraulic fluid pressure for operation, and typically receives this supply of line hydraulic fluid pressure from a hydraulic fluid pressure pump by way of a line pressure regulation valve which modifies the output hydraulic fluid pressure produced by the pump by releasing a part of said output pressure back to a hydraulic fluid reservoir through a release port.
Further, such an automatic transmission conventionally includes a fluid torque converter, which provides a fluid coupling between the engine of the vehicle and the gear transmission mechanism, thus eliminating the need for any clutch system for the drive train of the vehicle, and allowing for the vehicle to be stationary while the engine is turning at a low rotational speed at or close to the idling speed without the engine stalling, as well as providing torque multiplication by fluid flow in a per se well known way when the vehicle is being accelerated at relatively low speed and relatively low engine rotational speed. Many such torque converters are of course presently well known. Generally, such a torque converter comprises: a housing of a generally toroidal shape, on the inside of which there are formed a series of vanes which constitute a pump impeller, and fixed to a power input shaft; a tubrine member mounted within the housing as fixed to a power output shaft; and a stator member mounted within the housing via a one way brake on a fixed member. The housing of such a torque converter is kept filled with hydraulic fluid, which is pumped thereinto and is also drained therefrom as will be more fully explained later, and in a per se well known way the pump impeller, the stator member, and the turbine member cooperate, when the housing of the torque converter is thus filled with hydraulic fluid, to define a toroidal hydraulic fluid flow circulation system, circulation of hydraulic fluid around which in the general circulation fashion of a smoke ring is arranged to transfer torque in a conventional manner between the pump impeller and the turbine member of the torque converter.
This supply of hydraulic fluid for filling the torque converter is typically provided to the inside of the housing thereof via a first channel defined along or beside the central rotational axis thereof--in more detail, via a hole in one of the shafts passing along said central rotational axis or through a space defined between two concentric ones of such shafts; and the draining of hydraulic fluid from the torque converter is also typically performed in a similar manner, through a second such channel. The supply of hydraulic fluid is provided, generally in the art, from a torque converter hydraulic fluid pressure regulation valve which supplies a supply of hydraulic fluid at a regulated torque converter hydraulic fluid pressure, which is generally rather lower than the line hydraulic fluid pressure, to the torque converter.
Further, it has become more and more common nowadays for such a torque converter to be provided with a lock up clutch, i.e. a mechanical clutch which, when actuated, mechanically couples together the pump impeller and the turbine member of the torque converter with regard to their rotation, so that the above mentioned hydraulic torque transmission between the pump impeller and the turbine member no longer occurs or is relevant.
It is well known and conventional for such a lock up clutch to be engaged or disengaged according to the directions of supply and draining of the torque converter hydraulic fluid pressure to and from the interior of the housing of the torque converter. In other words, when the torque converter hydraulic fluid pressure mentioned above is being supplied to one channel which leads to the interior of the torque converter housing, and is being released from another channel, then it is arranged that the lock up clutch is engaged; and when the torque converter hydraulic fluid pressure is being supplied to said other channel, and is being drained from said one channel, then it is arranged that the lock up clutch is disengaged. Thus the supply of torque converter hydraulic fluid pressure to the torque converter from the torque converter pressure regulation valve is used for two purposes: to fill the torque converter with hydraulic fluid; and to selectively engage and disengage the lock up clutch, according to the direction of said supply.
The selective engagement of this lock up clutch is performed by a control device such as a hydraulic fluid pressure control device incorporated in the above mentioned hydraulic fluid pressure control system which controls the engagement of the various gear speed stages of the gear transmission mechanism, according to the operational conditions of the vehicle to which the torque converter incorporating this lock up clutch is fitted. In more detail, generally such a lock up clutch is desirably engaged when the torque converter is required to transmit rotary power at a fairly high rotational speed, at which time the torque conversion function of the torque converter is not substantially required. In such a case, if the lock up clutch is not engaged, then, although the torque converter at this time provides a substantially direct power transmission function between its pump impeller and its turbine member, nevertheless a small amount, such as a few percent, of slippage between the pump impeller and the turbine member will inevitably occur, and this will waste a substantial amount of energy because of the useless churning of hydraulic fluid within the torque converter, and also will cause undesirable heating up of the hydraulic fluid contained within the torque converter. On the other hand, such a lock up clutch is desirably, of course, disengaged when the road speed of the automobile is low, or when the rotational speed of the internal combustion engine thereof, i.e. the rotational speed of the pump impeller of the torque converter, is so low as to be close to idling rotational speed, in order to utilize the buffering action of the torque converter at these times, as well as the torque multiplication function thereof. Thus, such a lock up clutch is engaged by the above mentioned hydraulic fluid pressure control device, typically, when and only when the vehicle incorporating the torque converter is being driven at high road speed with the gear transmission mechanism in its highest gear speed stage, with the internal combustion engine of the vehicle thus operating at fairly high rotational speed, in which circumstances the actual hydraulic torque conversion function of the torque converter is not in fact particularly required. The provision of such a lock up clutch is effective for increasing fuel economy of the vehicle, especially when running on the open road such as on an expressway.
A well known prior art construction for such a hydraulic fluid pressure control device for controlling a lock up clutch has comprised (a) a lock up clutch control valve, which is switched between two positions, and which, when in its first switched position, switches said torque converter hydraulic fluid pressure mentioned above so as to supply it to said one channel which leads to the interior of the torque converter housing, and drains said other channel, so as to engage said lock up clutch, and which, when in its second switched position, switches said torque converter hydraulic fluid pressure mentioned above so as to supply it to said other channel, and drains said one channel, so as to disengage said lock up clutch, said lock up clutch control valve being switched to said first switched position thereof by supply of a control hydraulic fluid pressure, and, when said control hydraulic fluid pressure is not supplied, being switched to said second switched position thereof; and (b) a lock up clutch interrupt valve, which is located at an intermediate point of a hydraulic fluid conduit which conducts a supply of hydraulic fluid pressure from a steady hydraulic fluid pressure source, typically the line hydraulic fluid pressure, to the lock up clutch control valve as said control hydraulic fluid pressure. Thus, when the lock up clutch interrupt valve is in a switched state to allow passage of hydraulic fluid pressure therethrough, then the lock up clutch control valve is switched to its said first switched position, and accordingly the lock up clutch is engaged; and, when the lock up clutch interrupt valve is in a switched state to prevent passage of hydraulic fluid pressure therethrough, then the lock up clutch control valve is switched to its said second switched position, and accordingly the lock up clutch is disengaged.
Further, the lock up clutch interrupt valve has typically conventionally been constructed as a spool valve including a spool element, which is axially reciprocated between two switched positions, which is impelled towards its first switched position in which said lock up clutch interrupt valve allows passage of hydraulic fluid pressure therethrough by a supply of the conventionally produced governor hydraulic fluid pressure (which is representative of the road speed of the automobile) to a pressure chamber defined at its one end, and which is impelled towards its second switched position in which said lock up clutch interrupt valve prevents passage of hydraulic fluid pressure therethrough by the compression force of a compression coil spring, said compression force being of course predetermined.
According to such a construction, when the pressure value of the governor hydraulic fluid pressure is less than a predetermined pressure value, i.e. when the automobile road speed is less than a predetermined road speed value, then the force of the compression coil spring prevails over the governor hydraulic fluid pressure in its effect to move said spool element of said lock up clutch interrupt valve, and accordingly said spool element of said lock up clutch interrupt valve is switched to its said second switched position, in which said lock up clutch interrupt valve prevents supply of said source hydraulic fluid pressure to said lock up clutch control valve, and accordingly said lock up clutch control valve is switched to its said second switched position, in which the lock up clutch is disengaged; but, when the pressure value of the governor hydraulic fluid pressure is greater than a predetermined pressure value, i.e. when the automobile road speed is greater than a predetermined road speed value, then the governor hydraulic fluid pressure prevails over the force of the compression coil spring in its effect to move said spool element of said lock up clutch interrupt valve, and accordingly said spool element of said lock up clutch interrupt valve is switched to its first switched position, in which said lock up clutch interrupt valve allows supply of said source hydraulic fluid pressure to said lock up clutch control valve, and accordingly said lock up clutch control valve is switched to its said first switched position, in which the lock up clutch is engaged. Thus, the lock up clutch is engaged or disengaged, according as the vehicle road speed is above or below said predetermined road speed value.
Such a lock up clutch interrupt valve presents no substantial problems as far as concerns its switching action to transit from its said second switched position to its said first switched position, when the vehicle road speed is gradually rising and the force on said spool element of said lock up clutch interrupt value due to said governor hydraulic fluid pressure gradually rises, and in this case the spool element of said lock up clutch interrupt valve is smoothly switched to its said first switched position, substantially as soon as the road speed of said vehicle rises above said predetermined road speed value, and accordingly said lock up clutch is reliably engaged at this time; but, on the other hand, with regard to the switching action of said lock up clutch interrupt valve to transit from its said first position to its said second position, when the vehicle road speed is gradually dropping and the force on said spool element of said lock up clutch due to said governor hydraulic fluid pressure gradually drops, in this case there is a risk, especially after a long period of use of the transmission, or when the hydraulic fluid therein has become rather old and dirty, that said spool element of said lock up clutch interrupt valve may stick in its said first switched position, and not properly transit to its said second switched position, even though the road speed of said vehicle has dropped to substantially below said predetermined road speed value. If such sticking occurs, it is quite possible for the sticking condition to be maintained, even when the vehicle comes to a complete halt; and in this case, of course, since because the lock up clutch remains engaged the slipping function of the torque converter is no longer available, the internal combustion engine of the vehicle will stall.
Further, another problem concerned with the functioning of such a lock up clutch interrupt valve is as follows. Generally the above mentioned predetermined road speed value, at a vehicle road speed above which the lock up clutch is engaged, and at a vehicle road speed below which the lock up clutch is disengaged, will be set so high that the gear transmission mechanism of the automatic transmission of the vehicle will always be in its highest speed stage when the road speed of the vehicle is equal to or greater than said predetermined road speed value. This is because the shifting of the gear transmission mechanism of the automatic transmission between its speed stages, for example between its highest speed stage and its next to highest speed stage, will cause a severe transmission shock, if the lock up clutch is engaged at the time of shifting. In other words, the torque shock cushioning effect of the torque converter is important when the gear transmission mechanism of the automatic transmission changes its speed stage, and accordingly the above mentioned predetermined road speed value is set so high as to ensure that there should be no chance of the gear transmission mechanism shifting between its speed stages while the lock up clutch is engaged. However, in exceptional circumstances the gear transmission mechanism of the automatic transmission may be forced to shift from its highest speed stage to a lower speed stage, at a vehicle road speed higher than the predetermined road speed value; for example, when the driver of the vehicle forces such a shift, by moving the manual transmission shift lever of the vehicle from the "D" range to the "3" range. In such a case, if the lock up clutch is kept engaged, a severe transmission shock is liable to occur. This can produce various undersirable effects, such as shortening the operational life of the friction engaging mechanisms of said transmission as well as the various gears and other parts thereof, and perhaps causing premature failure of the automatic transmission as a whole, as well as damaging the drivability of the vehicle and perhaps even causing a dangerous accident.